The present invention relates to a multi-beam scanning optical system in which a plurality of laser beams emitted by a plurality of laser diodes are deflected to scan on a surface to be scanned such as a circumferential surface of a photoconductive drum.
A scanning optical system to be employed in a laser printer for forming a monochrome (e.g., a black-and-white) image is, for example, provided with a laser diode, which is driven in accordance with image data. The laser beam emitted by the laser diode is collimated by a collimating lens, and is directed to a deflecting member such as a polygonal mirror. The laser beam, which is incident on light reflecting surfaces of the rotating polygonal mirror, is deflected to scan within a predetermined angular range. The scanning laser beam is incident on an fxcex8 lens, refracted and converged thereby, and then is incident on an evenly charged photoconductive surface of a photoconductive drum to form a beam spot which moves along the rotational axis of the photoconductive drum (i.e., along a main scanning direction). Since the laser diode is driven in accordance with the image data, the surface of the photoconductive drum is exposed to light corresponding to the image data. While the light beam scans on the surface of the photoconductive drum, it is rotated (i.e., an auxiliary scanning is performed). Thus, a two-dimensional latent image is formed on the photoconductive surface of the photoconductive drum.
Then, toner is applied to the latent image to form a developed image, which is transferred onto a recording sheet and fixed thereon.
Recently, color laser beam printers, which is capable of forming a color image, has been developed. In the color laser printer, generally, a plurality of laser diodes are provided (which will be referred to as a multi-beam laser printer). Further, the corresponding number of fxcex8 lenses, and the corresponding number of photoconductive drums are provided for forming images of respective color components (e.g., yellow, magenta, cyan and black components). The above-described, exposing and developing processes are performed for each color component, and thus formed four color images (developed images) are transferred on a recording sheet to form a single color image and the color image is fixed.
In the multi-beam laser printer as described above, the plurality of laser diodes emit the laser beams having the same wavelength.
The plurality of laser diodes are held by holding members which are independently provided for holding respective laser diodes.
In the multi-beam scanning optical system as described above, however, the following problem may arise.
A laser diode includes a light emitting member which is driven to emit a laser beam, and a casing for accommodating the light emitting member. The light emitting member generates heat when it is driven to emit the laser beam. The light emitting member has a characteristic such that the wavelength of the emitting laser beam increases when the temperature of the light emitting member increases, and the wavelength decreases when the temperature decreases.
Therefore, if a plurality of laser diodes are driven for different durations at different timings, the temperature of the plurality of laser diodes may become different from each other. Then, the wavelengths of the laser beams emitted by the plurality of laser diodes may be different from each other.
An fxcex8 lens, on which the plurality of laser beams are incident, exhibits the same optical characteristics for the laser beams having the same wavelength. If the laser beam incident of the fxcex8 lens have different wavelengths, the optical characteristics are different for respective laser beams. If the laser beams have different wavelengths are incident on the fxcex8 lens, the beams passed through and emerged from the fxcex8 lens are incident on the photoconductive drum at different positions in the main scanning direction. In such a case, a color shift occurs in the color image formed on a recording sheet.
In view of the above problems, it is an object of the present invention to provide an improved multi-beam scanning optical system which is capable of preventing positional deviations of the beams, in the scanning direction, due to the difference of temperatures of the laser diodes.
For the above object, according to the present invention, there is provided a multi-beam scanning optical system, which is provided with a plurality of laser diodes respectively emitting a plurality of laser beams, each of the plurality of laser diodes being accommodated in a casing, and a laser diode holding unit that holds the casings of the plurality of laser diodes. heat generated by each of the plurality of laser diodes being mutually conducted among the plurality of laser diodes through the casings of the plurality of laser diodes and the laser diode holding unit.
Since the heat generated by individual laser diodes is mutually conducted, all the laser diodes have substantially the same temperature, and therefore, the shift of the scanning beam in the scanning direction due to the temperature difference can be prevented.
Preferably, the laser diode holding unit being formed of a single member made of material having relatively high heat conductivity.
Further preferably, the plurality of laser diodes emit laser beams having substantially the same wavelength at the same temperature.
Optionally, the multi-beam scanning optical system may further be provided with a polygonal mirror having a plurality of reflecting surface, the plurality of laser beam deflected by the polygonal mirror and scan within a predetermined angular range, and an optical system, the scanning beams deflected by the polygonal mirror being incident on the optical system, the optical system converging the incident beams on a plurality of surfaces to be scanned, respectively.
In this case, it is preferable that the optical system converges the incident beams at least in a direction parallel to the scanning direction of the incident beams.
Optionally, the optical system includes an fxcex8 lens system having a first lens, a second lens and a third lens, the plurality of beams passing through the first, second and third lenses in this order.
Further optionally, the second lens converges the beams passed therethrough in the main scanning direction.
Still optionally, the first lens converges the beams passed therethrough mainly in the auxiliary scanning direction.
Furthermore, the third lens may be provided for each of the plurality of beams, the third lens mainly converges a beam passed therethrough in the auxiliary scanning direction.
In particular, a plurality of third lenses may be provided at positions facing the plurality of surfaces to be scanned, respectively. optionally, the plurality of laser diodes include four laser diodes that respectively emit four light beams for forming four different color images.
In this case, the four different colors may include yellow, magenta, cyan and black.
Still optionally, the surfaces to be scanned include surfaces of a plurality of photoconductive drums for forming different color images, rotational axes of the plurality of photoconductive drums extending in the main scanning direction, the rotational axes of the plurality of photoconductive drums being parallel and apart from each other in the auxiliary scanning direction that is perpendicular to the main scanning direction.
Optionally, the plurality of laser diodes include four laser diodes that respectively emit four light beams for forming four different color images.
In this case, the four different colors may include yellow, magenta, cyan and black. The light source emits four light beams for forming the four different color images.
According to another aspect of the invention, there is provided a multi-beam laser printer for forming an image consisting of a plurality of color components in accordance with an electrophotographic imaging process. The printer is provided with a scanning optical system which includes a plurality of laser diodes respectively emitting a plurality of laser beams, each of the plurality of laser diodes being accommodated in a casing, a laser diode holding unit that holds the casings of the plurality of laser diodes, heat generated by each of the plurality of laser diodes being mutually conducted to each other through the casings of the plurality of laser diodes and the laser diode holding unit, a polygonal mirror having a plurality of reflecting surface, the plurality of laser beam deflected by the polygonal mirror and scan within a predetermined angular range, and an optical system, the scanning beams deflected by the polygonal mirror being incident on the optical system, the optical system converging the incident beams on a plurality of surfaces to be scanned, respectively.
According to the printer configured as above, since the heat generated by individual laser diodes is mutually conducted and finally leveled such that all the laser diodes have substantially the same temperature, the shift of the scanning beam in the scanning direction due to the temperature difference can be prevented.